The present invention relates to a rolling shaft, and in particular relates to the rolling shaft corresponding to an inner ring of a rolling bearing (in particular, a radial needle bearing).
Conventionally, as the rolling shaft corresponding to the inner ring of the radial needle bearing, such a material has been used that a through-hardening steel as SUJ2 which is further performed with a quenching and a tempering so as to provide a Vickers hardness being Hv650 or more. In this case, for providing the Vickers hardness of Hv650 or more, in response to processing requirements as a lathe turning, the rolling shaft having Vickers hardness Hv300 or less is performed to an outer periphery thereof with the lathe turning, and subsequently the outer periphery is performed with a high frequency quenching.
However, recently the radial needle bearing has been very often used under high load. In this case, a rolling fatigue resistance of the rolling shaft corresponding to the inner ring of the radial needle bearing is not sufficient with the conventional through-hardening steel such as SUJ2 or the ordinary high frequency-quenched steel. Major part of such insufficient rolling fatigue resistance is concerned with tolerance (surface fatigue resistance) against a surface fatigue which is caused in a surface of a raceway of the rolling shaft by contamination of a lubricant or supply shortage thereof.
In the radial needle bearing, such surface fatigue is caused in a portion from the surface thereof till a depth corresponding to 2% of the diameter Da of the rolling element (called said depth as xe2x80x9c2% Daxe2x80x9d hereafter), or in a portion from the surface till 0.1 mm in the depth of an absolute value, and in particularly a large surface fatigue occurs in a portion until 0.05 mm from the surface. In order to improve the surface fatigue resistance, it is necessary that a residual austenite is 15 to 40 vol % (preferably, 20 to 35 vol %) in the surface layer which is effected with fatigue. However, there is a problem that, for rendering the residual austenite to be 15 to 40 vol % (preferably 20 to 35 vol %) by the through-hardening or the ordinary high frequency-quenching, the quenching temperature must be heightened, whereby austenite crystal particles in a hardened part by quenching are coarsened, so that the surface fatigue resistance goes down.
It has been found out that, even if the residual austenite is present and a stress caused in the rolling shaft is within a limit of elasticity, the rolling shaft comprising the bearing steel (through-hardening) of SUJ2 which is worked under high load, is affected with a plastic deformation by a time-passing decomposition (transformation into martensite) of the residual austenite by the stress.
Further, the conventional rolling shaft is involved with another problem that it is made of a material (SC, SCr, SCM, SNCM) having Vickers hardness of Hv300 or less and carbon concentration of 0.4 wt % or less, which is carried out on the outer periphery with the high frequency-quenching (hardness of surface layer is Hv650 or more), and since part (core portion) other than the surface layer has hardness Hv300 or less and if a high impact load is given, the plastic deformation probably occurs.
It is, accordingly, an object of the invention to solve the problems of the conventional rolling shaft and provide a rolling shaft which is excellent in the rolling fatigue resistance and is least to generate the plastic deformation.
For solving the problems, the rolling shaft of the invention relatively rolling with respect to an opposite member, is characterized in that the rolling shaft comprises a steel containing 0.5 to 1.2 wt % carbon and is provided with a surface layer containing 0.05 to 0.4 wt % nitrogen, having Vickers hardness being Hv650 or more through a high frequency quenching performed thereto and a residual austenite being 15 to 40 vol %, and a core portion of the rolling shaft contains 0 vol % of the residual austenite.
If the rolling shaft is produced in the way that, after nitrogen is 0.05 to 0.4 wt % nitrified into the surface, the high frequency-quenching is performed to provide a surface layer having Vickers hardness of Hv650 or more and the residual austenite of 15 to 40 vol %, the residual austenite may be formed without coarsening the austenite crystal particles in the surface layer. Thus the rolling fatigue resistance may be increased in the raceway surface (the above mentioned surface layer).
Further, in case the rolling shaft is produced in the way that, after nitrogen is 0.05 to 0.4 wt % nitrified into the surface, the quenching and the tempering are performed to thermally refine the whole hardness of the rolling shaft to be Hv300 to 500 (preferably Hv400 to 500), and subsequently the high frequency-quenching is practiced to provide the surface layer having Vickers hardness of Hv650 or more and the residual austenite of 15 to 40 vol %, the rolling fatigue resistance in the raceway surface (the above mentioned surface layer) may be increased as in the above mentioned way, and at the same time, the hardness of the other part (core portion) than the surface layer may be made Hv300 to 500 (preferably Hv400 to 500) and the residual austenite in the other part (core portion) than the surface layer may be made 0 vol %, whereby the plastic deformation in the rolling shaft may be prevented, which is caused together with the time-passing decomposition of the residual austenite owing to the stress (stress within the elasticity limit) occurring in the rolling shaft, and further, the plastic deformation when adding the high impact load may be prevented.
The surface layer referred to in the invention is meant by the portion from the surface to 2% Da, or the portion from the surface until 0.1 mm (in particular 0.05 mm) in the depth of the absolute value.
Herein, critical meanings of these values will be described.
[Carbon Concentration in the Steel: 0.5 to 1.2 wt %]
If the carbon concentration is less than 0.5 wt %, it is difficult to obtain a stabilized hardness Hv650 (Hrc58) or more in the surface layer and in the high frequency-quenched portion. For obtaining the preferred hardness Hv650 (Hrc58) or more, regardless of the dimensions of the rolling shaft, the lower limit should be 0.5 wt %.
Further, when performing a carbonization by way of a carbonitriding process, 0.5 wt % or more of carbon is needed for forming fine (0.5 to 1.0 xcexcm) carbonitrides in the surface layer.
Further, if the carbon concentration is more than 1.2 wt %, macro carbides are easily produced in the steel to decrease a rolling life.
[Nitrogen Concentration in the Surface Layer: 0.05 to 0.4 wt %]
If nitrogen is made solid together with carbon in a quenched structure, it has an effect strengthening a matrix. From this fact, as the surface hardness is increased and the tempering resistibility is also increased, an abrasion resistance is available over a wide temperature range, so that the rolling shaft may have a long service life.
If the nitrogen concentration is less than 0.05 wt %, the abrasion resistance is insufficient and it is difficult to obtain 15 vol % or more of the residual austenite in the surface layer. Exceeding 0.4 wt %, it takes a long time for processing (polishing or grinding) after the heat treatment. A post processing cost is heightened, accordingly.
In order to make the abrasion resistance and the post processing cost optimum, preferable is 0.1 to 0.3 wt %.
Particularly, it is preferable that the surface layer for supporting the rolling faces and making difficult to generate the surface fatigue is made of the portion of 0.05 mm or more, or 2% Da from the surface of the completed rolling shaft, and the nitrogen concentration at the positions (the portion of more than 0.05 mm, or 2% Da from the surface) is 0.1 wt % or more, preferably 0.2 wt % or more.
[Hardness in the Surface Layer: Vickers Hardness Hv650 (Hrc58) or more]
If the hardness in the surface layer is less than Hv650, it is insufficient, so that the surface fatigue (rolling fatigue) is early produced, and the life of the rolling shaft is reduced.
[Hardness in the Core Part: Vickers Hardness Hv of 300 to 500]
If the Vickers hardness is less than Hv 300, the rolling shaft is easily generated with a yield, and the plastic deformation is apt to appear owing to the high impact load, so that warp of the rolling shaft is large. When the load acts thereon from the opposite members (needles in a case of the radial needle bearing) such as the rolling elements of the rolling bearing, the surface fatigue is locally produced, and consequently, the life of the rolling shaft is reduced.
Being more than Hv500, although the yield is less to occur but a toughness is reduced (rupture elongation is lowered), the rolling shaft might be broken, and therefore the Vickers hardness is preferably Hv400 to 500 from the viewpoint of the plastic deformation and the impact resistibility.
[Amount of the Residual Austenite in the Surface Layer: 15 to 40 vol %]
For example, if the needle bearing is used in transmission and engine driving system of automobiles, the surface fatigue easily occur due to foreign materials such as worn particles coming into the lubricant or shortage of lubricant supply. In the invention, other than hardening of the surface and providing of carbonitrides in the surface, it has been found that the surface fatigue may be reduced by a certain kind of a damper effect of the residual austenite.
If the residual austenite is less than 15 vol %, the damper effect is little for mitigating the surface fatigue. The life of the rolling shaft is, therefore, shortened. Being more than 40 vol %, the surface hardness is decreased so that the abrasion resistibility and the surface fatigue resistibility are rather detracted.
If the residual austenite is 20 to 35 vol %, a more excellent and stabilized life may be obtained.
[Amount of the Residual Austenite in the Core Portion: 0 vol %]
With presence of the residual austenite, the plastic deformation is caused by transformation into martensite. Although the amount of the residual austenite slightly influences the surface layer, however, since the core portion occupies a major part in volume of the rolling shaft, and if the residual austenite exists in the core portion, the rolling shaft is easily effected with the plastic deformation and the rolling shaft largely bends, and consequently, the fatigue strength goes down (owing to the local surface fatigue caused by the bending stress) Namely, if the residual austenite in the core portion is 0 vol %, the plastic deformation rarely occurs in the rolling shaft, though the residual austenite exists in the surface layer.
xe2x80x9c0 vol %xe2x80x9d may be made by thermally refining, otherwise a material including the xe2x80x9c0 vol %xe2x80x9d residual austenite may be used as it is. Thereby, the rolling shaft is not deformed by an external stress or a heat.
Thus according to the invention, by making the residual austenite in the core portion 0 vol % and making the hardness in the core portion Hv300 to 500, the rolling shaft is prevented from the plastic deformation due to an external force acting thereon. Further, by giving the hardness, the nitrogen amount and the residual austenite to the rolling element as well as the rolling surface, the rolling life is elongated.